1. Field of the Invention
The present invention relates to an optical spectrum analyzer, in particular, to a method and an apparatus for calibrating an optical spectrum analyzer in wavelength while reducing calibration errors.
2. Description of the Related Art
The conventional method of calibrating an optical spectrum analyzer in wavelength starts with the emission from a light source of light having a predetermined wavelength band, which is then collimated with a lens. The resulting parallel light is admitted into a gas absorption cell which is filled with a gas having such characteristics that it absorbs only a specified wavelength component of light. Hence, the incident light has a specified wavelength component absorbed by the gas in the absorption cell. This absorption wavelength is known and only one wavelength is preliminarily set as a reference value in a memory section.
The light passing through the gas absorption cell is condensed with a lens and launched into an optical spectrum analyzing section, where the spectrum of the light is obtained.
The information about the spectrum obtained in the optical spectrum analyzing section is output to a CPU section for executing calibration. The CPU section detects the wavelength having the largest absorption in the spectrum; that is, only a single wavelength in the wavelength component absorbed by the gas absorption cell is detected by the CPU section. The detected single wavelength is compared with the known wavelength preset as a reference value in the memory section to calculate the difference in wavelength, or the error in measurement. On the basis of the calculated difference in wavelength, the CPU section determines a calibration value which, in turn, is based to calibrate the optical spectrum analyzer.
On account of its inherent characteristics, the results of measurement with the optical spectrum analyzer are inevitably subject to variations within a certain range, producing wavelength linearity (a maximum and a minimum error of measurement) of the shape shown in FIG. 4. Obviously, the maximum in the wavelength linearity of the optical spectrum analyzer is 0.04 nm. Suppose here that the analyzer is calibrated with reference to xcexA, the minimum error of measurement. The difference in wavelength at point D in FIG. 4 is corrected and, after calibration, the errors of measurement at the respective wavelengths are as shown in FIG. 5, producing a maximum error of 0.04 nm.
Thus, the conventional method of calibration which is referenced to only one wavelength has had a serious defect in that depending on the reference wavelength, there occurs an error of measurement as great as 0.04 nm.
The present invention has been accomplished under these circumstances and has as an object providing a method of calibrating an optical spectrum analyzer in wavelength that produces a smaller amount of errors in measurement than the conventional method.
Another object of the invention is to provide an apparatus that can implement the above-mentioned method of calibrating an optical spectrum analyzer in wavelength.
A first object of the invention is attained by a method for calibrating an optical spectrum analyzer in wavelength, in which the light from a light source emitting light in a predetermined wavelength band is passed through a gas absorption cell to have light absorbed at specified wavelengths and calibration in wavelength is performed on the basis of the result of optical spectrum measurement conducted after the absorption, said method further including the steps of detecting the difference between the result of measurement of each of the wavelengths having dominant absorption in the optical spectrum obtained by said measurement after the absorption and a preset reference value, calculating the average of the detected differences, and performing calibration in wavelength on the basis of said average.
A second object of the invention is attained by an apparatus for calibrating an optical spectrum analyzer in wavelength, which comprises a reference light source emitting light in a predetermined wavelength band, a gas absorption cell that absorbs light at specified wavelengths as particular portions of the light emitted from said reference light source, a wavelength difference detecting means for detecting the difference in wavelength between the result of measurement of each of the wavelengths having dominant absorption in the spectrum of the light passing through said gas absorption cell and a preset reference value, and a calculating means which calculates the average of the wavelength differences detected by said wavelength difference detecting means, calibration in wavelength being performed on the basis of the average calculated by said calculating means.
The apparatus of the second aspect may be modified according to a third aspect of the invention, wherein said light source is a light-emitting diode issuing light having a predetermined wavelength band.
The apparatus of the second or third aspects may be modified according to a fourth aspect of the invention, wherein said gas absorption cell is filled with a gas having a plurality of absorption spectra.